1,4-Dialkynylbutatrienes: Synthesis, stability, and perspectives in the chemistry of carbo-benzenes

Article abstract of DOI:10.1002/chem.201002769

The π-electron-rich C₈-conjugated sequence of 1,4-dialkynylbutatrienes is identified as a fragile and fascinating motif occurring in carbo-benzene derivatives, and in Diederich's 1,4-bis(arylethynyl)- or 1,4-bis(triisopropylsilylethynyl)butatriene "capped" representatives, in particular, in tetraalkynylbutatriene. The family of symmetrical 1,4-dialkynylbutatrienes (E-C≡C)RC=C=C=CR(C≡C-E) is extended to functional caps (E=H, CH₃, C≡CPh, CPh=CHBr, or CPh=CBr₂) with non-alkynyl substituents at the sp² vertices (R=Ph or CF₃). The targets were selected for their potential in appealing retrosynthetic routes to carbo-benzenes, in which the aromatic C₁₈ macrocycle would be directly generated by sequential metathesis or reductive coupling processes. The functional 1,4-dialkynylbutrienes were synthesized by either classical methods used for the preparation of generic butatrienes (R′Li/CuX-mediated reductive coupling of gem-dihaloenynes or SnCl₂/HCl-mediated reduction of 3,6-dioxy-octa-1,4,7-triyne precursors). Their spectroscopic and electrochemical properties are compared and analyzed on the basis of the relative extent of total conjugation.

Full text of DOI:10.1002/chem.201002769

DOI: 10.1002/chem.201002769
1,4-Dialkynylbutatrienes: Synthesis, Stability, and Perspectives in the
Chemistry of carbo-Benzenes
Valꢀrie Maraval,*[a, b] Lꢀo Leroyer,[a, b] Aya Harano,[a, b, c] Cꢀcile Barthes,[a, b]
Alix Saquet,[a, b] Carine Duhayon,[a, b] Teruo Shinmyozu,[c] and Remi Chauvin*[a, b]
Abstract: The p-electron-rich C8-conju-
gated sequence of 1,4-dialkynylbuta-
trienes is identified as a fragile and fas-
cinating motif occurring in carbo-ben-
zene derivatives, and in Diederichꢀs
1,4-bis(arylethynyl)- or 1,4-bis(triiso-
propylsilylethynyl)butatriene “capped”
representatives, in particular, in tetraal-
kynylbutatriene. The family of symmet-
non-alkynyl substituents at the sp2 ver-
tices (R=Ph or CF3). The targets were
selected for their potential in appealing
retrosynthetic routes to carbo-ben-
zenes, in which the aromatic C18 macro-
cycle would be directly generated by
sequential metathesis or reductive cou-
pling processes. The functional 1,4-di-
ACHTUNGTRENNaUNG lkynylbutrienes were synthesized by
either classical methods used for the
preparation of generic butatrienes
(R’Li/CuX-mediated reductive cou-
pling of gem-dihaloenynes or SnCl2/
HCl-mediated reduction of 3,6-dioxy-
octa-1,4,7-triyne precursors). Their
spectroscopic
and
electrochemical
properties are compared and analyzed
on the basis of the relative extent of
total conjugation.
ꢀ ꢁ
rical 1,4-dialkynylbutatrienes (E C
Keywords: alkynes · allenes · aro-
maticity · conjugations · UV/Vis
spectroscopy
ꢁ ꢀ
C)RC=C=C=CRACHTUNGTRENNUNG(C C E) is extended
ꢁ
to functional caps (E=H, CH3, C
CPh, CPh=CHBr, or CPh=CBr2) with
Introduction
In their pioneering studies, Diederich, Houk, and co-
workers showed that the stability of di- and tetra-alkynylbu-
Disregarding the effects of aromaticity,[1] which are expected
to be small due to the size of the C18 macrocycle, the stabili-
ty of carbo-benzene rings[2] contrasts with the high reactivity
of “isolated” dialkynylbutatrienes without further p-conju-
gation beyond the sp carbon termini. These molecules con-
stitute models of the three intersecting one-edge-two-vertex
bis(ethyndiyl)butatriene C8 motifs that form the Kekule
structures of carbo-benzenic macrocycles with the same sub-
stituents at the sp2 vertices (Scheme 1).
tatrienes (E C C)RC=C=C=CR
the presence of substituents at the sp carbon atoms (R=C
ꢀ
G
ꢀ ꢁ
ꢁ ꢀ
2
C E versus H) and by suitable caps at the terminal sp
carbon atoms (E=aryl, trialkylsilyl, and tert-butyl).[3] The cis
and trans isomers were obtained in a ratio close to 1:1.5,
and the authors provided evidence for a surprisingly facile
cis/trans isomerization at room temperature (DG
ꢂ25 kcalmolꢀ1) with a low thermodynamic preference, as
expected from the weak steric interaction between the
remote substituents. More generally, isomerization of gener-
ic butatrienes is fundamentally facilitated by the bispropar-
gylic diACHTNUTRGNErUNG adical form of their valence bond (VB) description
[a] Dr. V. Maraval, Dr. L. Leroyer, Dr. A. Harano, C. Barthes,
A. Saquet, C. Duhayon, Prof. R. Chauvin
CNRS, LCC (Laboratoire de Chimie de Coordination)
205, route de Narbonne
(Scheme 1). This form is stabilized by the “aromaticity” of
the central triple bond (as compared with the bisallylic di-
AHCTUNGTRENNUNG
radical form of analogous 1,3-butadienes),[1] and correlates
31077 Toulouse (France)
with a low-lying open-shell singlet transition state (TS) of
the adiabatic stereoconversion though a perpendicular con-
figuration of the sp2 ends. The stabilization of both the
ground states and TS of stereo-interconverting butatrienes
naturally depends on the nature of the substituents, and it
was shown that the stabilizing effect of alkynyl substituents
is remarkably strong.
Known 1,4-dialkynylbutatrienes other than carbo-benzene
derivatives are limited to two capped versions (E=4-
Me2NC6H4, (iPr3)Si) of unsubstituted diethynylbutatriene
Fax : (+33)561-55-30-03
E-mail: vmaraval@lcc-toulouse.fr
chauvin@lcc-toulouse.fr
[b] Dr. V. Maraval, Dr. L. Leroyer, Dr. A. Harano, C. Barthes,
A. Saquet, C. Duhayon, Prof. R. Chauvin
Universitꢁ de Toulouse
UPS, INP, LCC, 31077 Toulouse (France)
[c] Dr. A. Harano, Prof. T. Shinmyozu
Institute for Materials Chemistry
and Engineering (IMCE)
and Department of Molecular Chemistry
Graduate School of Sciences
Kyushu University, Hakozaki 6-10-1 Higashi-ku
Fukuoka 812-8581 (Japan)
(R=H), which are less stable than the corresponding tet-
[3a]
ꢁ ꢀ
raalkynylbutatrienes (R=C C E). The paucity of exam-
ples of 1,4-dialkynylbutatrienes thus calls for an investiga-
tion of the stability limit versus the nature of both the cap E
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002769.
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Chem. Eur. J. 2011, 17, 5086 – 5100

FULL PAPER
1,4-Bis(triisopropylsilylethynyl)-
1,4-diphenylbutatriene (2; R=
Ph, E=TIPS): The TIPS-
capped reference molecule of
this study (2) was prepared by
the classical method used for
the preparation of other TIPS-
capped alkynylbutatrienes, con-
sisting of a reductive homo-cou-
pling of gem-dibromo olefinic
precursors.[3] The parent ketone
3 was obtained in two steps
from benzaldehyde and triiso-
propylsilylacetylene via the
propargylic alcohol 4. Dibro-
mo-olefination of 3 was per-
formed by a recently described
Scheme 1. Dialkynylbutatrienes as isolated models of the three intersecting one-edge-two-vertex bis(ethyn-
diyl)butatriene C8 motifs constituting the Kekule structure of carbo-benzene rings. Their electronic and stereo-
chemical features, as well as the two main reductive retrosynthetic approaches, are illustrated.
and substituents R (Scheme 1). Caps with minimal steric
and electronic demand deserve specific attention for their
possible selective reactivity. For example, non-capped di-
procedure involving triisopropylphosphite instead of the
classically used triphenylphosphane reagent, and afforded
the gem-dibromoenyne 5a in 89% yield (Scheme 2, top).[5]
This enyne was then treated with butyllithium at ꢀ1008C,
ACHTUNGTRENNUNGethynylbutatrienes (E=H) could undergo deprotonation
and electrophilic functionaliza-
tion, such as Eglington–Glaser
oxidative homocoupling or So-
nogashira coupling with aro-
matic halides.[3b] Methyl-capped
diACHTUNGTRENNUNGalkynylbutatrienes (E=CH3)
can also be regarded as “func-
tional” for homologous depro-
tonation/electrophilic substitu-
tion processes or alkyne meta-
thesis (see below). Beyond (and
related to) the carbo-benzene
series, the family of functional
Scheme 2. Alternative syntheses of the TIPS-capped 1,4-diethynyl-1,4-diphenylbutatriene 2 through either
gem-dibromoenyne 5a (top) or gem-dichloroenyne 5b (bottom).
1,4-diACHTUNGTRENNUNGalkynylbutatrienes is here-
after extended to representa-
tives stabilized by non-alkynyl
substituents, in particular, with R=Ph or CF3. Their synthe-
sis is undertaken by either classical methods used for the
preparation of generic butatrienes,[4] that is, R’Li/CuX-medi-
ated reductive coupling of gem-dihaloalkenes, or SnCl2/HCl-
mediated reduction of 1,4-dioxy-but-2-yne precursors
(Scheme 1).
and then with nBu3P·CuI,[6] which is a soluble copper salt
that was first used by Iyoda and later by Diederich to pre-
pare butatrienes in quite good yields.[3,7] This method afford-
ed the targeted dialkynyldiphenylbutatriene 2 in 65% isolat-
ed yield as a 50:50 mixture of cis and trans isomers (accord-
1
ing to integration of the o-CH H NMR signals of their re-
spective phenyl groups).
In the search for methodological improvements, the same
butatriene derivative 2 was also targeted through the analo-
gous gem-dichloroenyne 5b. The latter was prepared by a
procedure described by Stachulski and co-workers,[8] which
is based on the use of a mixture of CCl4 and PPh3 in aceto-
nitrile as a specific solvent. This method was reported to be
particularly efficient with ketone substrates, and here afford-
ed the gem-dichloroenyne 5b in 88% yield from ketone 3
(Scheme 2, bottom). Treatment of 5b with nBuLi, and then
with copper cyanide, led to the formation of butatriene 2 in
poorly reproducible yields (50–95%) depending on the qual-
ity of the CuCN sample. Replacement of the latter salt by
tri-n-butylphosphane-coordinated copper iodide resulted in
Results and Discussion
An uncapped diethynylbutatriene: Diederich and co-work-
ers reported that protodesilylation of tris(isopropylsilyl)
(TIPS)-capped 1,4-diethynylbutatrienes (with two other al-
kynyl substituents R) did not allow for the characterization
of any product.[3b] Because no uncapped version of 1,4-di-
ACHTUNGTRENNUNGethynylbutatriene has been described, a phenyl-substituted
representative 1 was targeted starting from the TIPS-capped
possible precursor 2.
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5087

V. Maraval, R. Chauvin et al.
excellent and reproducible yields (94%). As illustrated in
Scheme 2, the route through the “greener” dichloroalkene
intermediate 5b is thus more efficient than the classical
route using the dibromo analogue 5a and the same
Bu3P·CuI reagent (Scheme 2).
Although two distinct spots were observed on the TLC
plate, the stereoisomers of 2 could not be separated by silica
gel chromatography. However, the mixture (an intense
yellow oil) crystallized progressively when kept at room
temperature for a few hours. X-ray diffraction analysis of
the crystals showed that they were composed of the pure
trans isomer of 2 (Figure 1).
at d=1.21 ppm, two signals at d=3.68 and 3.73 ppm in a
60:40 ratio were assigned to the acetylenic protons of the cis
and trans isomers of 1 (without configurational assignment).
All attempts to record a clean 13C NMR spectrum of 1
failed due to its poor stability in sufficiently concentrated
solutions. Nevertheless, 1 could be kept for a few days at
48C in pentane (approximately 3 mm, obtained after filtra-
tion through silica gel with pentane as eluent).
A methyl-capped diethenylbutatriene: Methyl groups are in-
teresting caps for the 1,4-diethynylbutatriene motif
(Scheme 1, E=CH3) not only because of their low steric
protecting effect, but also because of their functional versa-
tility. Beyond the possibility of propargylic deprotonation
and electrophilic functionalization, 1,4-dipropynylbuta-
trienes might undergo sequential metathesis to generate vol-
atile but-2-yne and 1,4-poly(triacetylenes) (1,4-PTA). The
latter are the chain carbo-mers of polyacetylenes, and the
regioisomers of classical 1,2-poly(triacetylenes) (PTA=1,2-
PTA), which have attracted considerable interest for their
promising conducting and optical properties.[9] Ring-closing
sequences of n such metathesis processes might ultimately
produce carbo-[2n]annulenes (see Scheme 4 for n=3). In
particular, while benzene can be obtained by cyclotrimeriza-
tion of acetylene,[10] carbo-benzene might result from cyclo-
trismetathesis of 1,4-dipropynylbutatriene. Guided by our
long-time ambition to develop novel strategies for the prep-
aration of carbo-benzene derivatives, the synthesis of 1,4-di-
propynylbutatriene key precursors was thus undertaken. Be-
cause all carbo-benzenes known to date have been obtained
by reductive aromatization of hexaoxy[6]pericyclynes,[1,2b,11]
the phenylated precursor 6 was targeted by reduction of the
Figure 1. ORTEP diagram of the trans isomer of dialkynylbutatriene 2.
corresponding 3,6-methoxyocta-1,4,7-triyne precursor
(Scheme 4, R=Ph).
7
ꢀ
ꢀ
ꢀ
Distances (ꢃ): C1 C1’=1.2440(13); C1 C2=1.3509(9); C2 C3=
ꢀ
ꢀ
ꢀ
1.4249(9); C3 C4=1.2156(10); C2 C5=1.4719(10); C4 Si1=1.8401(7);
angles (8): C1’-C1-C2=179.43(9); C1-C2-C3=117.80(6); C2-C3-C4=
178.47(8); C3-C4-Si1=177.35(17); C1-C2-C5=121.58(6); C3-C2-C5=
120.63(6).
The synthesis of the dioxytriyne 7 was envisaged through
double addition of acetylide derivatives to dibenzoylacety-
lene 8, itself prepared from commercial trans-dibenzoyl-
AHCTUNGTRENNUNG
ethene using a known two-step procedure.[12] Direct addition
of two equivalents of propynylmagnesium bromide to 8,
however, failed to produce the triynediol 9, only polymeric
materials was obtained (Scheme 5).
Synthesis of a diethynylbutatriene (E=H, R=Ph): Deprotec-
tion of the ethynyl arms of 2 was performed with tetra-
A
The triyne diether 7 was then targeted from the known
at low temperature, affording 1 as the first characterized ex-
ample of a non-capped diethynylbutatriene (Scheme 3). This
compound appeared to be quite unstable, and polymerized
instantaneously upon concentration. However, after flash
purification of the crude reaction mixture through silica gel,
bis
by addition of two equivalents of lithium trimethylsilylacet-
ylide to the diketone 8.[13] Cleavage of the trimethylsilyl
groups of 10 led to the formation of the bis(terminal) triyne
11 in 79% yield. However, the simultaneous quadruple
methylation of 11 proved to be problematic. Under these
conditions, in addition to the formation of several products
resulting from partial methyla-
ACHTUNGTREN(NUNG silyl)-protected triynediol 10, which was readily prepared
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1
the H NMR spectrum of a diluted solution of pure 1 could
be recorded. In the absence of the intense TIPS signals of 2
tion or from other side-re-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
isolated in 23% yield only after
a tedious chromatographic pu-
rification. A more efficient syn-
thesis of 7 was finally achieved
Scheme 3. Desilylation of the TIPS-capped diethynylbutatriene 2 to the uncapped version 1.
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1,4-Dialkynylbutatrienes
FULL PAPER
Treatment of 7 (either as the
meso/dl mixture or as the pure
meso isomer) with SnCl2/HCl
was carried out at low tempera-
ture in CH2Cl2 (Scheme 6). At
ꢀ408C, an intense yellow col-
AHCTUNGTERGoNNUN ration of the reaction mixture
appeared progressively, suggest-
ing the formation of the buta-
triene
chromophore.
Two
yellow spots with very similar
Rf values of similar intensity
were observed by TLC analysis
(AcOEt/n-heptane, 1:9), which
were attributed to the cis and
trans isomers of 1,4-dipropynyl-
butatriene 6. After neutraliza-
tion of the medium by addition
of 1n NaOH and washings with
water, the organic layer was
separated. When concentrated
to dryness, the dipropynylbuta-
triene 6 appeared to be unsta-
ble. However, it could be kept
in CH2Cl2 at 48C for several
weeks without decomposition.
The methyl-capped diethynyl-
butatriene 6 thus appears to be
more stable than its uncapped
counterpart 1 (Scheme 3). The
remaining sensitivity of 6 in the
dry amorphous state, however,
prevented the determination of
an accurate yield. Nevertheless,
when the reaction was per-
formed in a CDCl3/DCl/D2O
mixture, NMR spectroscopic
analysis of the CDCl3 layer
showed that the triyne diether 7
was completely converted and
that 6 was the sole product.
This was indicated by the obser-
vation of deshielded CH3
1H NMR signals at d=2.23 and
Scheme 4. Retrosynthetic design of carbo-polyacetylenes (=1,4-poly(triacetylenes) or 1,4-PTAs) and related
carbo-[2n]annulenes (carbo-benzene for n=3) based on sequential metathesis of the key target of this study,
1,4-dipropynyldiphenylbutatriene 6.
Scheme 5. Alternative synthetic routes to the triyne diether 7.
through an alternative, one-step longer route involving
double O-methylation of the diol 10 followed by desilylation
of the resulting diether 12, giving the bisACHTNUTRGNEUNG(terminal) triyne
2.26 ppm (the corresponding protons of both isomers of 7
resonate at d=1.98 ppm) and by the disappearance of the
OCH3 signals (the produced methanol being transferred
into the aqueous layer). The two propynyl CH3 1HNMR sig-
nals integrated in a 1:1 ratio, showing that the cis and trans
isomers were formed in equal amounts.
Although the cis and trans isomers of 6 could not be sepa-
rated by column chromatography, crystals were deposited by
slow evaporation of concentrated CH2Cl2 solutions of the
mixture at low temperature, and X-ray diffraction analyses
showed that they corresponded to the pure trans-6 isomer
(Figure 3). The carbon skeleton of the molecule appears to
be almost perfectly planar in the solid state. This butatriene
13.[13] Double C-methylation of 13 was achieved by treat-
ment with n-butyllithium and iodomethane, giving the ex-
pected triyne-diether 7 in good yield (Scheme 5). The final
product and all intermediates 10–13 were obtained as mix-
tures of meso and dl diastereoisomers. One pure stereoiso-
mer of 7 could be separated by selective crystallization from
hexane, and X-ray diffraction analysis indicated that the
crystalline sample corresponded to the meso isomer
(Figure 2), as previously observed for several other octa-
1,4,7-triyn-3,6-diol derivatives.[14]
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5089

V. Maraval, R. Chauvin et al.
Figure 3. ORTEP diagram of the trans isomer of the dipropynylbuta-
ꢀ
ꢀ
ꢀ
triene 6. Distances (ꢃ): C1 C1’=1.240(3); C1 C2=1.3425(17); C2 C3=
ꢀ
ꢀ
ꢀ
1.425(2); C3 C4=1.189(2); C4 C5=1.452(2); C2 C6=1.4736(18);
angles (8): C1’-C1-C2=177.50(19); C1-C2-C3=119.90(12); C2-C3-C4=
176.66(15); C3-C4-C5=177.20(17); C1-C2-C6=121.24(12); C3-C2-C6=
118.85(11).
Figure 2. ORTEP diagram of the meso isomer of the triyne-diether 7.
Distances (ꢃ): C1-C1’=1.189(2); C1-C2=1.4863(16); C2-C3=
1.4756(15); C3-C4=1.1853(15); C4-C5=1.4518(16); C2-O1=1.4183(14);
C6-O1=1.4208(16); C2-C7=1.5317(16); angles (8): C1’-C1-C2=
178.43(16); C1-C2-C3=108.96(9); C2-C3-C4=178.35(13); C3-C4-C5=
179.65(15); C1-C2-O1=110.79(9); O1-C2-C3=111.29(9); O1-C2-C7=
107.85(9); C3-C2-C7=109.15(9); C1-C2-C7=108.74(9); C2-O1-C6=
114.85(10).
ture (for E=TIPS) and the isomerization barrier in CD2Cl2
of which was estimated to be approximately 25 kcal
molꢀ1.[3c,15] These observations can be explained by the
effect of the phenyl substituents of 6, which stabilize the di-
AHCTUNGTREGUNrNN adical form of the isomerization TS (Scheme 1).
The dipropynylbutatriene 6 is
the first example of a fully char-
acterized
diethynylbutatriene
motif that is weakly protected
by caps other than the aryl, tert-
butyl, or trialkylsilyl groups.
Moreover, the reductive elimi-
nation route from the corre-
sponding but-2-yne-1,2-diol di-
ether proved to be compatible
with the intrinsically reactive
propynyl substituents.
Scheme 6. Reduction of the triyne diether 7 (as either a meso/dl mixture or the pure meso isomer) to 1:1 mix-
tures of cis- and trans-1,4-diphenyl-1,4-dipropynylbutatriene (6).
In preliminary experiments,
thus adopts the configuration in which the steric repulsions
are minimized, and the conformation in which the conjuga-
tion is most the efficient, with respect to coplanar phenyl
rings. All attempts at recording the 1H NMR spectrum of
pure trans-6, however, failed because of rapid isomerization
in solution at room temperature; ten minutes after the prep-
aration of the NMR sample in CDCl3, a 70:30 trans-6/cis-6
mixture was observed, and, after one hour, a steady 50:50
ratio was reached. The corresponding approximate first-
order kinetic constant can thus be estimated as k=
however, the substrate 6 was found to be surprisingly inert
when submitted to alkyne metathesis conditions using stan-
dard catalysts,[16] even at high temperature (e.g., in toluene
at 1108C). Systematic studies using more sophisticated cata-
lysts are thus planned.[17]
Alkenyl- and alkynyl-capped diethynylbutatrienes: The
main challenge in the chemistry of carbo-benzenes is the
design of a direct method allowing for the formation of the
C18 macrocycle in its aromatic form, and thus circumventing
the problematic step of reductive aromatization of [6]pericy-
clynic precursors (Scheme 4). In parallel to the metathesis
approach aiming at the simultaneous formation of three
butyne edges from the dipropynylbutatriene 6 (see above),
an alternative ternary sequential route to hexaphenyl-carbo-
benzene 15 would consist of the simultaneous formation of
three butatriene edges from the gem,gem-tetrahalodienynes
14a or 14b (Scheme 7). Methodological investigations were
first undertaken for the preparation of these key substrates
from dibenzoylacetylene 8.
0.00076 sꢀ1, which, in the Eyring model, corresponds to a
¼
free enthalpy of activation DG
G
22 kcalmolꢀ1. This value is remarkably close to the rotation-
al energy barrier calculated at the UB3LYP/6-31G(d) level
of theory for the unsubstituted diethynylbutatriene in the
¼
gas phase (R=E=H: DG
N
diphenyl derivative 6 (R=Ph, E=Me), however, appears to
be configurationally less stable than the related unsubstitut-
ed dialkynylbutatrienes (R=H, E=4-Me2NC6H4, TIPS), the
trans isomer of which could be resolved at room tempera-
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1,4-Dialkynylbutatrienes
FULL PAPER
(Table 1, entry 6).[20] All at-
tempts
gem,gem-tetrachlorodienyne
analogue 14b using PPh3 and
chloromethane reactants were
at
preparing
the
AHCTUNGTRENNUNG
unsuccessful (Table 1, entries 7–
10).[21] Addressing these diffi-
culties, a modification of the
phosphane reagent was envi-
sioned. The use of HMPT was
Scheme 7. Alternative ternary sequential approaches to hexaphenyl-carbo-benzene 15 (see also Scheme 4).
The key tetrahalodienyne substrates 14a and 14b of the reductive coupling approach (right) might be pre-
pared from dibenzoylacetylene 8.
reported
by
Jung
and
DꢀAmico,[22] and was, in this
case, found to afford 14a in a
slightly better yield (21%) than
Synthesis of gem,gem-tetrahalodienynes: The gem,gem-tetra-
bromodienyne 14a was first targeted by applying the “classi-
cal” PPh3/CBr4 system reported by Ramirez and co-workers
in 1962.[18] This method, which had been optimized for alde-
hyde substrates, was found to convert the diketone 8 into
tetrabromodienyne 14a in 19% yield only. Alternative gem-
dihalo-olefination procedures were thus attempted; the re-
sults are summarized in Table 1.
obtained with triphenylphosphane (Table 1, entries 11 vs. 1).
Very recently, Lautens and co-workers compared the effi-
ciency of various phosphane reagents, including trialkylphos-
phanes and phosphites.[23] For dibromo-olefination of 8, trib-
utylphosphane was found to be inefficient (Table 1,
entry 12), but triisopropylphosphite gave the tetrabromodi-
AHCTUNGTREGUNeNN nyne 14a in a much improved yield of 70% (Table 1,
entry 13).
Alternative multistep methods for gem-dibromo-olefina-
tion were attempted from diketone 8. The first approach,
described by Rezaei and Normant, proceeds in three
steps.[24] The diol 16 was prepared from 8 by addition of two
equivalents of the lithium salt of bromoform, and then sub-
mitted to trans-esterification with isopropenyl acetate to
give the diester 17 in 75% yield (Scheme 8).
Addition of ethylmagnesium bromide to 17, however,
failed to produce the tetrabromodienyne 14a, and afforded,
instead, the dibromodienyne 18 as a single Z,Z-stereoisomer
in 42% yield. The structure of 18 was confirmed by X-ray
diffraction analysis of a single crystal (Figure 4). As in the
case of the dipropynylbutatriene 6 (Figure 3), the carbon
skeleton of the dienyne 18 was perfectly planar, with an op-
timal conjugation of the phenyl rings allowed by a 1,4-trans-
oid conformation.
Table 1. Comparative procedures for double gem-dihalo-olefination of
dibenzoylacetylene 8 (Scheme 7).
Entry
Reactants
Solvent
Yield
in 14 [%]
1
2
3
4
5
PPh3/CBr4
PPh3/CBr4
PPh3/CBr4
PPh3/CBr4 (preformed ylide)
Ph3P+CHBr2,Brꢀ/tBuOK
PPh3/CBr4/Zn
CH2Cl2
19
<5
<5
19
0
toluene
1,2-dichloroethane
CH2Cl2
CH2Cl2
CH2Cl2
6
7
13
0
PPh3/CCl4
CH2Cl2
8
9
10
11
12
13
PPh3/CCl4 (preformed ylide)
PPh3/CHCl3/tBuOK
PPh3/CCl4/Mg sonication
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
0
0
0
21
0
P
P
P
A
R
A
70
Improvements in the use of
triphenylphosphonium reac-
tants were first investigated.
Whereas dichloromethane was
found to be an optimal solvent
for the generation of the tetra-
bromodienyne 14a (Table 1, en-
tries 2 and 3), pre-formation of
the dibromomethylphosphoni-
Scheme 8. Attempted procedure described by Rezaei and Normant for the three-step gem-dibromo-olefination
of dibenzoylacetylene 8.[24]
um ylide reactant from CBr4 and PPh3 did not result in any
improvement (Table 1, entry 4),[19] and the use of alternative
ylide precursors (Ph3P+CHBr2, Brꢀ and tBuOK) failed com-
pletely (Table 1, entry 5).[19] A known procedure, allowing
for a decrease in the amount of triphenylphosphane used by
addition of zinc, was also attempted. Under these condi-
tions, however, 14a was obtained in a lower yield of 13%
A two-step, gem-dibromo-olefination procedure reported
by Nenadjenko and co-workers[25] was also attempted. This
approach consists of the condensation of hydrazine with a
carbonyl precursor, followed (possibly in one-pot) by treat-
ment with CBr4 and a copper salt (Scheme 9). In the case of
the diketone substrate 8, however, the expected dihydrazone
did not form, and only the monohydrazone 19 could be iso-
Chem. Eur. J. 2011, 17, 5086 – 5100
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5091

V. Maraval, R. Chauvin et al.
ditions were employed (Scheme 10). Two copper salts were
tested and, in both cases, trace amounts of the carbo-ben-
zene 15 could be detected by its characteristic apolar purple
spot on the TLC plate, along with several other chromo-
phoric products. The formation of 15 (in approximately 1%
yield or less) was confirmed by its characteristic signals in
1
the H NMR spectrum of the crude material (at d=9.45 and
3
7.99 ppm, J
N
ic purification of 15, however, failed. Nevertheless, new
chromophoric butatrienes could be separated by preparative
HPLC. The less polar product eluting after 15 was found to
correspond to a tetra-debromo-dimer of 14a according to
MS analysis, and the carbo-cyclobutadiene structure 20 was
thus initially proposed. This hypothesis was, however, ruled
out by the NMR data, which displayed two types of none-
quivalent phenyl groups, and the structure was finally as-
signed to the bis(butadiynyl)bu-
ꢀ
Figure 4. ORTEP view of the dibromodienyne (Z,Z) 18. Distances (ꢃ):
ꢀ
ꢀ
ꢀ
ꢀ
C1 C1’=1.196(4); C1 C2=1.430(3); C2 C3=1.342(3); C2 C4=
ꢀ
ꢀ
1.486(2); C3 Br1=1.866(2); C3 H31=0.961; angles (8): C1’-C1-C2=
178.9(3); C1-C2-C3=120.66(17); C1-C2-C4=117.61(16); C3-C2-C4=
121.74(17); C2-C3-Br1=123.12(15); C2-C3-H31=122.291; Br1-C3-H31=
114.594.
tatriene 21 (Scheme 10). The
pure product (which was also
present as
a component in
other HPLC fractions) was iso-
lated in 2% yield as a 55:45
mixture of cis and trans stereo-
isomers (without assignment),
and was found to be surprising-
ly stable at room temperature.
The formation of 21 formally
Scheme 9. Attempted procedure described by Nenadjenko and co-workers for the two-step gem-dibromo-olefi-
nation of dibenzoylacetylene through a hydrazone intermediate.[25]
results from
three reactions:
a
sequence of
monocou-
a
lated (in 66% yield). The latter compound was found to be
totally unreactive towards either hydrazine or the CuCl/
NH3/CBr4 reagent; an observation which could be explained
by hydrogen-bonding effects.
pling of two molecules of tetrabromodienyne 14a, generat-
ing the butatriene moiety, followed by two Fritsch–Butten-
berg–Wiechell (FBW) rearrangements,[26] affording the buta-
diyne moieties from the remaining vinylidene functions.
The more polar, disymmetric 1,4-dialkynyl-butatriene 22
was also separated by HPLC. Although it could not be iso-
lated in pure form (samples contained ca. 10% of impurity),
the structure of 22 was confirmed by MS and NMR analy-
ses, which indicated a mixture of four diastereoisomers
(their ratio could, however, not
Coupling of gem,gem-tetrahalodienynes: The tetrabromo-
ACHTUNGTRENNUNGdienyne 14a was submitted to the reductive coupling condi-
tions previously described for the preparation of the dialky-
nylbutatriene 2. To prevent polymerization, quite dilute con-
be determined from the com-
plex 1H NMR spectrum). This
product, which was isolated in
5% yield, can be considered as
an intermediate in the forma-
tion of 21, in which only one
FBW rearrangement occurred,
while hydrolysis of the second
lithiumbromovinylidene lead to
the bromoolefinic group of 22.
On the basis of MS data, the
impurity present in the isolated
sample of 22 was shown to cor-
respond to the tetrabromo-de-
rivative 23.
1,4-Bis(trifluorometyl)dialky-
Scheme 10. Dehalo-coupling of the tetrabromodienyne 14a.
nylbutatrienes: In the weakly
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Chem. Eur. J. 2011, 17, 5086 – 5100

1,4-Dialkynylbutatrienes
FULL PAPER
polar dialkynylbutatrienes described above, or their dienyne
parents, the central p system is formally cross-conjugated
with two phenyl substituents. The perfectly planar confor-
mation of the representatives 6 and 18 in the crystal state
(Figures 3 and 4) thus suggests that the phenyl rings may
significantly contribute to their chemical stability. To evalu-
ate the cross-conjugation effect, replacement of the phenyl
groups by trifluoromethyl groups of similar conical size[27]
but without a p system was envisaged. The s-withdrawing
ability of the CF3 groups was, however, expected to induce
concentration (and thus polarization) of the p-electron
cloud above the electron-deficient s system of the buta-
triene unit. Overall stabilization was envisaged to occur with
TIPS caps in the fluorinated target 24.
The synthesis of 24 was first attempted by reductive elimi-
nation from the 1,4-dioxybut-2-yne 25. Following a de-
scribed procedure,[28] the ketone precursor 26 was prepared
in 91% yield by addition of lithium triisopropylsilylacetylide
to ethyl trifluoroacetate in the presence of boron trifluoride
(Scheme 11). Addition of ethynylmagnesium bromide to 26
Figure 5. ORTEP diagram of the d/l isomer of the fluorinated triynediol
ꢀ
ꢀ
ꢀ
25. Distances (ꢃ): C1 C2=1.531(5); C2 C3=1.472(5); C3 C4=
ꢀ
ꢀ
ꢀ
1.197(5); C4 Si1=1.840(4); C2 C5=1.477(5); C5 C5’=1.187(5); angles
(8): C5-C2-C3=109.4(3); C1-C2-C5=108.6(3); C1-C2-C3=109.3(3); C2-
C5-C5’=174.8(4); C2-C3-C4=178.5(4).
This result can be explained by
the difficulty of protonation of
the trifluoromethylated alcohols
to generate the corresponding
destabilized carbenium ion. The
same conditions were thus ap-
Scheme 11. Synthesis of the fluorinated dioxytriynediol 25.
plied after complexation of one
of the triple bonds of 25 with a
[Co2(CO)6] unit; eliminative re-
at 08C afforded the bis(propargylic) alcohol 27 in 85%
yield. Finally, treatment of 27 with two equivalents of n-bu-
tyllithium followed by one equivalent of ketone 26 gave the
fluorinated dioxytriynediol 25 in 71% yield.
The triynediol 25 was obtained as a colorless oil consisting
of a statistical mixture of two diastereoisomers. One of the
diastereoisomers progressively crystallized at room tempera-
ture, and its X-ray diffraction analysis showed that it corre-
sponded to the d/l isomer (Figure 5). This result is quite sur-
prising, because all the previously described X-ray diffrac-
tion structures of related crystalline dioxytriyne derivatives
were found to correspond to the meso isomer.[14]
The fluorinated triynediol 25
duction of 1,4-dioxybut-2-yne moieties by stabilization of
the propargylic carbenium intermediates as Nicholas com-
plexes was indeed previously reported in the particular case
of a hexaalkynylhexaoxy[6]pericyclyne.[30] The targeted bu-
tatriene derivative 24 was, however, not observed, and the
triyne precursor 25 was recovered after treatment with cer-
ium(IV) ammonium nitrate (CAN). To explain this failure,
it can be proposed that complexation of the [Co2(CO)6] unit
occurred at the central, less hindered, triple bond (the exter-
nal triple bonds are hindered by the bulky TIPS groups),
and could thus block the elimination process, even if the
cobalt-stabilized carbenium ion was produced (Scheme 12).
was then submitted to carefully
chosen reductive elimination
conditions. Among various re-
agents that have been proposed
for this transformation, the
SnCl2/HCl system proved to be
efficient for generation of the
dipropynylbutatriene
6
(Scheme 6), and was thus se-
lected.[29] However, no reaction
occurred and 25 was quantita-
tively recovered after neutrali-
zation and filtration of the re-
Scheme 12. Attempted preparation of the butatriene 24 by reductive elimination from the dioxytriyne 25, and
ACHTUNGTRENNUNG
action mixture (Scheme 12). proposed explanation for the failure of the cobalt method.[30] [Co]=[Co2(CO)6].
Chem. Eur. J. 2011, 17, 5086 – 5100
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5093

V. Maraval, R. Chauvin et al.
An alternative route based on the reductive coupling of
vinylidene moieties was then envisioned. The preparation of
butatrienes bearing perfluoro-alkyl or -aryl groups was pre-
viously reported from the corresponding gem-dibromo-ole-
finic precursors.[31] gem-Dibromo-olefination of the ketone
26 described above (Scheme 11) was first carried out using a
mixture of PPh3 and CBr4, affording the dibromoenyne 28
in 82% yield (Scheme 13).
the cap E is indeed negligible, with 13C NMR chemical shifts
between d=148.0 and 149.0 ppm for the same substituent
R=Ph (Table 2, entries 2 to 4).
In contrast, the influence of the R substituents for a given
cap E (=TIPS) gives rise to a marked variation in the
13C NMR shifts over a range of more than 10 ppm (Table 2,
entries 4–7). For p-cross-conjugated substituents (R=Ph or
ꢁ
C C-TIPS), the butatrienic sp carbon atoms thus resonate
at d=149–151 ppm (Table 2, entries 4 and 7), whereas a s-
electron-withdrawing substituent (R=CF3) induces an ex-
pected deshielding and a consequent shift to d=158.5 ppm
(Table 2, entry 5). It is worth noting that the most deshield-
ed signal is observed for the unsubstituted dialkynylbuta-
triene (R=H) described by Diederich and co-workers
(Table 2, entry 6; d=161.6 and 161.3 ppm).[3a]
For comparison, the all equivalent sp carbon atoms of
hexaphenyl-carbo-benzene 15 resonate out of this range, at
Scheme 13. Synthesis of the dialkynylbis(trifluoromethyl)butatriene 24.
Reductive coupling of 28 was
attempted by using the proce-
dure described by Burton et al.,
which takes place through a
zinc intermediate formed at
room temperature under soni-
cation, and treatment with a
copper salt in situ at low tem-
perature.[31,32] This method af-
forded the fluorinated buta-
triene 24 as the major product
in the crude reaction mixture
(Scheme 13). However, purifi-
cation of this apolar product
proved to be difficult, and it
could be isolated in a pure state
Table 2. 13C NMR chemical shifts of the central sp carbon atoms (ppm, in CDCl3), and maximum absorption
ꢀ ꢁ
wavelengths [nm, in CHCl3 unless otherwise noted] of symmetrical 1,4-dialkynylbutatrienes (E C C)RC=C=
ꢁ ꢀ
C=CR
Entry
(Compd)
G
R
E
d
N
lmax (l’)
Reference
AHCTUNGTRENNUNG
1 (1)
2 (6)
3 (21)
4 (2)
5 (24)
6
Ph
Ph
Ph
Ph
CF3
H
H
Me
426
437
499
453
this work
this work
this work
this work
this work
[3b]
148.4
148.0, 148.2
149.0
158.5
161.6, 161.3
150.6
ꢁ ꢀ
C C Ph
Si
Si
Si
Si
G
E
373
N
356[a]
424[b]
472 (515)
ꢁ ꢀ
C C Si
Ph
7
G
G
[3a]
[11]
8 (15)
(C(Ph)C2)31
118.5
[a] Recorded in CH2Cl2. [b] Recorded in hexane.
in only 22% yield after silica gel chromatography (eluted
with pentane). The product was obtained as a 40:60 mixture
of cis and trans isomers (without assignment), as indicated
by integration of the two signals observed by 19F NMR anal-
ysis at d=ꢀ64.1 and ꢀ64.3 ppm.
d=118.5 ppm (Table 2, entry 8), and are thus quite far from
the corresponding resonance (ca. d=148 ppm) of acyclic di-
alkynylbutatriene hydrocarbons such as 2 and 21 (Table 2,
entries 2 and 3). This is clearly due to the partial dialkynyl-
butatriene Lewis character of the edges of the C18 macrocy-
cle, which is mesomerically hybridized with a dialkenyl-
The chemical stability of 24 thus shows that cross-conjuga-
tion of the 1,4-dialkynylbutatriene motif with phenyl sub-
stituents is not a necessary condition. Desilylation of 24 was
expected to afford a highly unstable and volatile product,
and was thus not attempted. Nevertheless, the information
gained on 24 suggests the stability of challenging trifluoro-
methylated carbo-benzene molecules.
AHCTUNGTREGaNNNU cetylene character. The pure latter character is found in
acyclic diphenyl dienynes, for which the central sp-carbon
atoms resonate at approximately 95 ppm (d=96.3 ppm for
14a and d=94.6 ppm for 18). The average chemical shift of
the central sp carbon atoms of diphenyldienynes and diphe-
nyldialkynylbutatrienes is thus equal to 1/2ACTHNUGTRENUNG(95+148)
ꢃ121.5 ppm, which is very close to the value observed for
hexaphenyl-carbo-benzene 15. This is a quantifiable conse-
quence (and confirmation) of the 50:50 contributions of the
two Kekule forms of the aromatic carbo-benzene macrocy-
cle.
Comparative physicochemical properties of dialkynylbuta-
trienes
13C NMR spectroscopy: The 13C NMR chemical shift of the
central sp carbon atoms of symmetrical 1,4-dialkynylbuta-
ꢀ ꢁ
ꢁ ꢀ
trienes (E C C)RC=C=C=CR
G
Absorption spectroscopy: The UV/Vis absorption spectra in
chloroform of the five dialkynylbutatrienes 1, 2, 6, 21, and
24, display the same general profile, with an intense maxi-
mum absorption band and two shoulders at shorter wave-
cis or trans configuration, but is expected to be primarily in-
fluenced by the nature of the substituents R at the sp2 verti-
ces. According to the data listed in Table 2, the influence of
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Chem. Eur. J. 2011, 17, 5086 – 5100

1,4-Dialkynylbutatrienes
FULL PAPER
even higher wavelength of l’(15)=515 nm (Table 2).[11b]
Conversely, reducing the extent of p-conjugation of 2 by re-
placement of the phenyl substituents with trisopropylsilyl-
AHCTUNGTREGeNNNU thynyl substituents (Table 2, entry 7), induces a moderate
hypsochromic shift (from 453 nm in CHCl3 for 2, to 424 nm
in hexane for Diederichꢀs TIPS-capped tetraethynylbutatri-
AHCTUNGTRENNUNG
ene).[3a] Complete interruption of cross-conjugation by the
introduction of trifluoromethyl substituents at the butatriene
vertices of 24, induces a dramatic hypsochromic shift of
80 nm (from 453 for 2 to 373 nm for 24). Nevertheless, the
hypsochromic effect of CF3 remains lower by approximately
20 nm than that observed for the corresponding hydrogen-
substituted compound (Table 2, entry 6).[3b]
Voltammetry: The electrochemical properties of cis/trans
mixtures of the dialkynylbutatrienes 1, 2, 6, 21, and 24 were
investigated by square-wave voltammetry (SWV) and cyclic
voltammetry (CV) in dichloromethane solution (10ꢀ3 m),
Figure 6. Absorption UV/Vis spectra of the four dialkynylbutatrienes 1,
2, 6, 21, and 24 (CHCl3). See also Table 2.
length (Figure 6, Table 2). The absorption coefficients of 2,
21, and 24 (which are stable in the solid state) are equal to
53440, 7000, and 40250 molLꢀ1 cmꢀ1, respectively. The
phenyl-substituted dialkynylbutatrienes 1, 2, and 6, with the
same extent of p conjugation, exhibit similar lmax values (be-
tween 426 and 453 nm), indicating that the alkyl, silyl, or H
caps have a weak influence on the electronic transitions. In-
creasing the extent of conjugation through the phenylethyn-
yl caps of 21 induces a bathochromic shift of 45 nm from
lmax (2)=453 nm to lmax (21)=499 nm. By comparison,
with [nBu4N]ACTHUNGTERNNU[G PF6] as a supporting electrolyte (0.1m). The
potentials listed in Table 3 are given versus the ferricinium/
ferrocene couple (Fc+/Fc) as internal standard. According
to SWV, all the dialkynylbutatrienes 1, 2, 6, 21, and 24 un-
dergo two to four reduction processes, and at most one oxi-
dation step that occurs between 0.75 and 0.97 V. CV experi-
ments at 0.1 Vsꢀ1 indicated that only the first reduction cou-
ples between ꢀ1.17 and ꢀ1.73 V are reversible.
The three phenylated butatrienes 1, 2, and 6 (R=Ph) pos-
sess the same degree of p conjugation (over 20 sp or sp2
carbon centers, limited by saturated caps E=H, Me, TIPS),
and behave similarly, with a single reversible couple be-
hexaACHTUNGTRENNUNGphenyl-carbo-benzene 15 exhibits a similar pattern at
lmax (15)=472 nm, with a secondary absorption band at an
Table 3. SWV and CV electrochemical data of dialkynylbutatrienes in CH2Cl2 with [nBu4N]
versus the Fc+/Fc couple as internal standard. CV: scan rate=100 mVsꢀ1
ACHTUNGTREN[NNUG PF6] as a supporting electrolyte (0.1m). Potentials are given
.
Compd
R
E
CV
SWV
0 red
1=2
[d]
ox
1=2
red
1=2
Eopx [V][a]
E
[V][b]
DEred [mV][c]
RIp
Erped [V][e]
ꢀ2.01
E
[V][f]
E
[V][g]
1
Ph
H
1.0
ꢀ1.52
ꢀ1.57
ꢀ1.73
ꢀ1.27
68
83
98
53
1.12
0.99
1.13
1.17
0.97
0.90
0.75
ꢀ1.52
ꢀ1.97
ꢀ2.20
ꢀ2.47
ꢀ1.57
ꢀ2.02
ꢀ2.10
ꢀ230
2
6
Ph
Ph
Si
(iPr)3
0.90[h]
0.78
ꢀ2.14
Me
ꢀ2.27
ꢀ2.41
ꢀ1.66
ꢀ1.73
ꢀ2.14
ꢀ2.29
ꢀ2.42
ꢀ1.29
ꢀ1.66
ꢁ
21
Ph
C C-Ph
0.88
–
0.88
–
ꢀ1.66
ꢀ1.17
63
107
1.21
1.00
24
CF3
Si
A
–
ꢀ1.17
ꢀ1.64
ꢀ2.20
–
29[i]
C C-Ar
Si
ACHTUNGERTN(NUNG iPr)3
1.03
ꢀ1.28
80
–
ꢀ1.78
–
ꢁ
[a] First oxidation peak potential determined by CV. [b] Half-wave potential of the first reversible reduction process determined by CV, E1/2 =[Epc +Epa]/
2, the arithmetic mean of the corresponding anodic and cathodic peak potentials. [c] CV peak-to-peak separations Ep(backward)ꢀEp(forward) =EpaꢀEpb, for the
first reversible reduction process. [d] Absolute current ratio jIp(backward)/Ip(forward) j=jIpa/Ipc j for the first reversible reduction process measured by CV.
[e] Reduction peak potential determined by CV for other reduction processes. [f] Half-wave potential of the first oxidation process determined by SWV.
[g] Half-wave potentials of all reduction processes determined by SWV. [h] The oxidation process becomes reversible at scan rate of 1 Vsꢀ1. [i] CV data
for the tetraalkynylbutatriene hydrocarbon with Ar=3,5-(tBu)2C6H4 (Scheme 14).[3b]
Chem. Eur. J. 2011, 17, 5086 – 5100
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5095

V. Maraval, R. Chauvin et al.
tween ꢀ1.52 V and ꢀ1.73 V, and three irreversible processes
in the range ꢀ1.97/ꢀ2.47 V. Reduction of 6 is, however,
more difficult because of the pure s-inductive effect of the
methyl cap (the s-inductive effect of the TIPS caps of 2 is
stronger, but is balanced by the silicon a-effect stabilizing
With respect to the phenylated analogue 2, two opposite
effects of the CF3 substituents of 24 on the corresponding
anions could be expected: 1) a destabilization by a reduction
in the extent of p-conjugation (from 20 to 8 carbon centers),
and 2) a stabilization by the s-withdrawing effect of the
electronegative fluorine atoms. Evidence that the latter
effect overcomes the former was obtained by observing a
cathodic shift of 0.4 V (ꢀ1.57 V for 2, ꢀ1.17 V for 24). Ac-
cording to SWV, the second (irreversible) reduction poten-
tial is shifted by the same amount (from ꢀ2.02 V for 2 to
ꢀ1.64 V for 24).
·
ꢀ
ꢀ
the reduced form -C=C Si). The uncapped diethynylbuta-
triene 1 exhibits the smallest absolute first reduction poten-
tial, but the reversibility of this step is a further illustration
of its chemical stability (see Figure 7 and section on the syn-
Conclusion
Six 1,4-dialkynylbutatrienes substituted with phenyl or tri-
fluoromethyl groups at the sp2 vertices have been synthe-
sized and characterized. Whereas 1,4-diphenyl-1,4-diethynyl-
butatriene 1 is the first uncapped diethynylbutatriene that
has been characterized to date (including by voltammetry),
the five other products can be considered to be functionally
capped 1,4-diethynylbutatrienes at the sp termini. Their ac-
cessibility and chemical stability open up new prospects for
their use in the scaffolding of phenylacetylene building-
blocks, especially in the synthesis of phenyl-substituted
carbo-benzene rings. The dynamics of cis/trans equilibria
might even be useful for the thermodynamic control of the
scaffolding under dilute conditions. Systematic studies are
currently focused on the reactivity of 1,4-diphenyl-1,4-pro-
pynylbutatriene 6 and diphenyltetrabromodienyne 14a in
alkyne metathesis and vinylidene coupling reactions, respec-
tively.
Figure 7. Cyclic voltammogram of the uncapped diethynylbutatriene 1
(10ꢀ3 molLꢀ1) in CH2Cl2
+ ACHTUNTGENRU[GN PF6] on Pt microdisk
0.1 molLꢀ1 [nBu4N]
(r=0.25 mm) at room temperature. Scan rate: 0.1 Vsꢀ1. Potentials were
measured versus SCE.
thesis above). Extending the p conjugation to 36 carbon
centers through the phenylethynyl caps of 21 results in an
easier reversible reduction at ꢀ1.27 V, and in the replace-
ment of the three irreversible reductions of 1, 2, and 6 with
a second reversible process at ꢀ1.66 V.
The first reduction potentials of the diphenyl-dialkynylbu-
tatrienes 1, 2, and 6 are consistently lower than those of
aryl- and silyl-capped tetraethynylbutatrienes reported by
Diederich and co-workers (in the range ꢀ1.28/ꢀ1.41 V).[3b]
This can be explained by the difference in the extent of p
conjugation through the phenyl substituent (6C) and ethy-
nylaryl substituents (8C). Nevertheless, the behavior and
the first peak reduction potential values of 21 (with a 36-
carbon p-conjugation extent) are almost identical to those
of the tetraalkynylbutatriene hydrocarbon 29 (E1/2 =ꢀ1.28,
ꢀ1.78 V), with a p-conjugation extent over 24 carbon cen-
ters only (Scheme 14). This illustrates the specific stabilizing
effect of silyl caps and/or the tetraethynylbutatriene core on
the anions.
Experimental Section
General: THF and diethyl ether were dried and distilled over sodium/
benzophenone; pentane and dichloromethane over P2O5. All other re-
agents were used as commercially available. In particular, commercial
2.5m solutions of nBuLi in hexane, and 0.5m solutions of ethynylmagnesi-
um bromide in THF were used. Previously described procedures were
used for the preparation of 8,[12] 10, 12, and 13.[13,14] Experimental details
for compounds 3, 4, 16, 17, 18, 19, 26, and 27 are available in the Sup-
porting Information. All reactions were carried out under nitrogen or
Argon using Schlenk and vacuum line techniques. Column chromatogra-
phy was carried out with silica gel (60 P, 70–200 mm). Silica gel thin-layer
chromatography plates (60F254; 0.25 mm) were developed by treatment
with an ethanolic solution of phosphomolybdic acid (20%) or an aqueous
potassium permanganate solution for fluorinated derivatives.
Bruker ARX 250, DPX 300, Avance 300, or Avance 400 spectrometers
were used; all NMR spectra were recorded as CDCl3 solutions. NMR
chemical shifts (d) are given in ppm, with positive values to high frequen-
cy relative to the tetramethylsilane reference for 1H and 13C spectra, and
CCl3F for 19F spectra; coupling constants (J) are given in Hz. Mass spec-
tra were recorded with a Quadrupolar Nermag R10–10H spectrometer.
UV spectra were recorded with
a Perkin–Elmer UV/Vis Win-Lab
Lambda 35 spectrometer. HPLC were recorded with Semi-preparative
Waters Deltaprep chains, coupled with a UV 486 Waters detector and a
Gibson 201 fraction collector (column: Sunfire Si 150ꢄ19 mm; elution
with petroleum ether/chloroform, 95:5; 15 mLminꢀ1). Voltammetric
measurements were carried out with a potentiostat Autolab PGSTAT100
Scheme 14. Known tetraalkynylbutatriene hydrocarbon with an extended
24-carbon p-conjugation system (see electrochemical data in Table 3).[3b]
5096
www.chemeurj.org
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 5086 – 5100

1,4-Dialkynylbutatrienes
FULL PAPER
instrument controlled by GPES 4.09 software. Experiments were per-
formed at RT in a homemade, airtight, three-electrode cell connected to
a vacuum/argon line. The reference electrode consisted of a saturated cal-
omel electrode (SCE) separated from the solution by a bridge compart-
ment. The counter electrode was a platinum wire of ca. 1 cm2 apparent
surface area. The working electrode was either a Pt microdisk (0.5 mm
diameter) or a glassy carbon microdisk (1 mm diameter). The supporting
o- or m-C6H5), 128.5 (s, p-C6H5), 128.7 (s, o- or m-C6H5), 131.2 (s, i-
ꢁ
˜
C6H5), 138.0 ppm (s, C=CBr2); FTIR: n=2942–2864 (Csp3-H), 2137 (C
C), 1462, 1443 cmꢀ1 (C=C); MS (DCI/NH3): m/z: 441.8 [M+H]ꢀ; HRMS
(DCI/CH4): m/z calcd for C19H26SiBr2: 440.0171 [M]+; found: 440.0178.
Synthesis of 5b: PPh3 (2.490 g, 9.49 mmol) and CCl4 (0.46 mL,
4.75 mmol) were added to a solution of 3 (0.680 g, 2.37 mmol) in anhy-
drous CH3CN (40 mL) at 08C. After stirring for 2 h at RT, the orange so-
lution was diluted with diethyl ether, washed with water and with brine,
dried over MgSO4, and evaporated to dryness. After purification by silica
gel chromatography (AcOEt/pentane, 5:95), the dichlorovinylidene de-
electrolyte [nBu4N]ACHTUNGTRENNUNG[PF6] (Fluka, 99% electrochemical grade) was used as
received and simply degassed under argon. Dichloromethane was freshly
distilled prior to use. The solutions used during the electrochemical stud-
ies were typically 10ꢀ3 m in butatriene and 0.1m in supporting electrolyte.
Before each measurement, the solutions were degassed by bubbling Ar,
and the working electrode was polished with a polishing machine (Presi
P230). Typical instrumental parameters for recorded square-wave voltam-
mograms were: SW frequency f=20 Hz, SW amplitude Esw =20 mV, and
scan increment DE=0.5 mV. All reported potentials are referenced to
the formal potential of the ferrocenium/ferrocene (Fc+/Fc) couple mea-
sured in the same electrolyte (ca. (0.45ꢄ0.02) V vs. SCE).
Synthesis of 1: TBAF (0.500 mL, 0.50 mmol) was added at ꢀ788C to a
solution of 3 (0.100 g, 0.18 mmol) in THF (5 mL). The resulting mixture
was stirred for 30 min at the same temperature, then treated with water.
After extraction of the aqueous layer with diethyl ether, the combined
organic layers were washed with brine and dried over MgSO4. The sol-
vent was partially evaporated to give a 5 mL solution, which was rapidly
purified by silica-gel chromatography (pentane). CDCl3 was added
before removal of most of the pentane under controlled vacuum to give
a yellow solution of 1 that was analyzed spectroscopically. 1H NMR
rivative 5b was isolated as
a
colorless oil in 88% yield. 1H NMR
(CDCl3): d=1.64 (s, 21H; iPr), 7.40–7.43 (m, 3H; m-C6H5 and p-C6H5),
7.57 ppm (m, 2H; o-C6H5); 13C{1H} NMR (CDCl3): d=11.3 (s, Si-CH-
ꢁ
ꢁ
CH3), 18.7 (s, Si-CH-CH3), 101.1 (s, C-Si), 103.4 (s, C C-Si), 124.0 and
128.1 (2 s, C=CCl2), 128.2 (s, o-C6H5 or m-C6H5), 128.5 (s, p-C6H5), 128.9
(s, o-C6H5 or m-C6H5), 135.8 ppm (s, i-C6H5); MS (DCI/NH3): m/z: 352.1
[MꢀH]+, 370.1 [M+NH4]+.
Synthesis of 6: SnCl2 (10 equiv., 1.66 g, 8.77 mmol) was added to a solu-
tion of 7 (0.300 g, 0.877 mmol) in anhydrous CH2Cl2 (50 mL) at ꢀ308C.
After stirring for 10 min at the same temperature, HCl·OEt2 (20 equiv.,
2m in Et2O, 8.77 mL, 17.54 mmol) was added. The resulting mixture was
stirred until complete disappearance of 7 was observed (reaction moni-
tored by TLC; ca. 2 h), then the reaction was neutralized by addition of
aqueous 1 N NaOH solution. The intense yellow organic layer was
washed three times with water and dried over MgSO4. The resulting solu-
tion was filtered through silica gel (CH2Cl2) and concentrated under
vacuum to approximately 20 mL (this compound must be kept in solution
and at low temperature). Slow evaporation of a CH2Cl2 concentrated so-
lution of 6 gave crystals of its trans isomer. The same reaction was per-
formed in an NMR tube using CDCl3 as solvent and DCl in D2O as re-
agent to characterize the compound. 1H NMR (CDCl3): d=2.25 (2 s, 6H;
ꢁ
(CDCl3): d=3.69, 3.75 (2ꢄs, 2H; C-H), 7.36–7.46 (m, 6H; m- and p-
C6H5), 7.84–7.87 (m, 4H; o-C6H5). UV/Vis: lmax =426 nm.
Synthesis of 2 from 5a: nBuLi (180 mL, 0.45 mmol) was added to a solu-
tion of 5a (0.200 g, 0.45 mmol) in anhydrous THF (10 mL) under stirring
at ꢀ1008C. After stirring the resulting solution for 1 h at ꢀ1008C, a solu-
tion of CuI·PBu3 (0.091 g, 0.23 mmol) in anhydrous THF (5 mL) was
added. This mixture was slowly warmed to RT and stirring was main-
tained overnight. The solution was filtered through 5 cm of silica gel,
eluting with diethyl ether, and, after evaporation to dryness, the residue
was purified by silica gel chromatography (pentane) to give 2 as a yellow
solid in 65% yield.
ꢁ
C-CH3), 7.31–7.44 (m, 6H; m- and p-C6H5), 7.82–7.86 ppm (m, 4H; o-
13
1
ꢁ
C6H5); C{ H} NMR (CDCl3): d=5.2, 5.3 (2 s; C-CH3), 77.2, 78.0 (2 s;
ꢁ
ꢁ
C C-CH3), 95.8, 96.0 (2 s; C C-CH3), 103.8, 104.3 (2 s; C-Ph), 127.2,
127.4 (2 s; o-C6H5), 128.5 (s; p-C6H5), 128.4, 128.5 (2 s; m-C6H5), 137.0,
137.4 (2 s; i-C6H5), 148.4 ppm (s; C=C=C=C). UV/Vis: lmax =437 nm.
Synthesis of 7: nBuLi (4.45 mL, 11.13 mmol) was added to a solution of 9
(1.59 g, 5.06 mmol) in anhydrous THF (40 mL) under stirring at ꢀ788C.
The resulting mixture was stirred for 50 min at ꢀ788C, then MeI
(0.95 mL, 15.18 mmol) was added and stirring was continued for 30 min
at ꢀ788C and for 75 min at RT. The mixture was diluted with diethyl
ether and treated with saturated aqueous NH4Cl. The aqueous layer was
extracted two times with diethyl ether, and the combined organic layers
were washed with brine, dried over MgSO4, and evaporated under re-
duced pressure. The residue was purified by silica gel chromatography
(AcOEt/pentane, 1:9) to give 7 as a crude sticky oil in 95% yield. A
sample of pure meso isomer was obtained by selective crystallization in
pentane from the mixture of diastereoisomers. 1H NMR (CDCl3): d=
Synthesis of 2 from 5b: nBuLi (225 mL, 0.56 mmol) was added to a solu-
tion of 5b (0.200 g, 0.56 mmol) in anhydrous THF (10 mL) under stirring
at ꢀ908C. After stirring the resulting solution for 1 h at ꢀ908C, a suspen-
sion of CuCN (0.025 g, 0.283 mmol) or a solution of CuI·PBu3 (0.114 g,
0.23 mmol) in anhydrous THF (5 mL) was added at ꢀ1008C. This mix-
ture was slowly warmed to RT and stirring was maintained overnight.
The solution was filtered through 5 cm of silica gel, eluting with diethyl
ether, and, after evaporation to dryness, the residue was purified by silica
gel chromatography (pentane) to give 2 as a yellow solid (50–91% with
CuCN and 94% with CuI·PBu3). 1H NMR (CDCl3): d=1.21 (m, 42H;
CH-CH3), 7.30–7.50 (m, 6H; m-C6H5 and p-C6H5), 7.85–7.87 ppm (m,
4H; o-C6H5); 13C{1H} NMR (CDCl3): d=11.4 (s, Si-CH-CH3), 18.7 (s, Si-
ꢁ
1.98 (s, 6H; C-CH3), 3.53 (s, 6H; OCH3), 7.37–7.40 (m, 6H; m- and p-
C6H5), 7.77–7.81 ppm (m, 4H; o-C6H5); 13C{1H} NMR (CDCl3; mixture
ꢁ
ꢁ
CH-CH3), 102.0 and 102.3 (2ꢄs, Si-C ), 104.1 (s, Ph-C), 104.5 and 104.9
meso/dl): d=3.81 (s; C-CH3), 53.07 (s; OCH3), 72.01 (s; C-OCH3), 76.95
ꢁ
ꢁ
ꢁ
(2ꢄs, Si-C C), 127.2 and 127.4 (2ꢄs, o-C6H5), 128.5 and 128.6 (2ꢄs, m-
and 76.97 (2ꢄs; C-CH3), 83.71 and 83.72, 84.51 and 84.53 (4ꢄs; C-C-
OCH3), 126.61 and 126.62 (2ꢄs; o-C6H5), 128.36 (s; m-C6H5), 128.69 (s;
p-C6H5), 140.84 and 140.86 ppm (2ꢄs; i-C6H5); 13C{1H} NMR (CDCl3;
C6H5), 128.8 (s, p-C6H5), 136.3 and 136.9 (2ꢄs, i-C6H5), 149.1 ppm (s, -C=
ꢁ
˜
C=C=C-); FTIR: n=2941–2864 (Csp3-H), 2127 (C C), 1548, 1488, 1461,
1383, 1265 cmꢀ1 (C-Si); UV/Vis: lmax (e)=453 nm (53440 molLꢀ1 cmꢀ1);
MS (DCI/NH3): m/z: 565.3 [M+H]+; HRMS (DCI/CH4): m/z calcd for
C38H53Si2: 565.3686 [M+H]+; found: 565.3698.
pure meso): d=3.8 (s; C-CH3), 53.0 (s; OCH3), 72.0 (s; C-OCH3), 76.9
(s; C-CH3), 83.6 and 84.4 (2 s; C-C-OCH3), 126.6 (s; o-C6H5), 128.3 (s;
m-C6H5), 128.6 (s; p-C6H5), 140.7 ppm (s; i-C6H5); FTIR: n˜ =2922 (Csp3
H), 2822 (OCsp3-H), 2239 (C C), 1599, 1489, 1449 (C=C), 1062 cm (C-
O); MS (DCI/CH4): m/z: 311.1 [MꢀOMe]+, 296.1 [MꢀOMeꢀMe]+;
HRMS (DCI/CH4): m/z calcd for C23H19O: 311.1436 [MꢀOCH3]+;
found: 311.1441.
ꢁ
ꢁ
ꢁ
-
ꢀ1
ꢁ
Synthesis of 5a: A solution of triisopropylphosphite (1.2 mL, 4.87 mmol)
in CH2Cl2 (5 mL) was added at 08C to a mixture of ketone 3 (0.465 g,
1.62 mmol) and CBr4 (0.810 g, 2.43 mmol) in CH2Cl2 (10 mL). After stir-
ring for 1.5 h, the solution was treated with saturated aqueous NaHCO3.
The aqueous layer was extracted with diethyl ether, and the combined or-
ganic layers were washed with brine, dried over MgSO4 and evaporated
to dryness. The residue was purified by silica gel chromatography (pen-
Synthesis of 14a: A solution of PACHTUNGTRENUNG(OiPr)3 (1.26 mL, 5.12 mmol) in CH2Cl2
(2 mL) was added at 08C to a solution of 8 (0.200 g, 0.85 mmol) and
CBr4 (0.850 g, 2.56 mmol) in CH2Cl2 (6 mL). The reaction mixture, which
became instantaneously red, was stirred for 1 h at 08C and then for 1 h at
RT. After treatment with aqueous saturated NaHCO3 and extraction
with diethyl ether, the combined organic layers were washed with brine,
dried over MgSO4, and evaporated under reduced pressure. The obtained
1
tane) to give 5a as a white solid in 89% yield. H NMR (CDCl3): d=1.12
(s, 21H; Si-CH-CH3), 7.37–7.40 (m, 3H; m- and p-C6H5), 7.49–7.53 ppm
(m, 2H; o-C6H5); 13C{1H} NMR (CDCl3): d=11.2 (s, Si-CH-CH3), 18.6 (s,
ꢁ
ꢁ
Si-CH-CH3), 99.6 (s, Si-C C-), 101.2 (s, CBr2), 105.2 (s, Si-C C), 128.2 (s,
Chem. Eur. J. 2011, 17, 5086 – 5100
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5097

V. Maraval, R. Chauvin et al.
white crystals were washed with ethanol and dried to give 14a in 70%
yield. M.p. 1548C; 1H NMR (CDCl3): d=7.37–7.42 (m, 6H; m- and p-
C6H5), 7.47–7.51 ppm (m, 4H; o-C6H5); 13C{1H} NMR (CDCl3): d=96.3
CHBr and m-C6H5-C=C=C=), 129.2, 129.3, 129.4, 129.59, 129.64 (p-
ꢁ
C6H5), 130.3, 130.4 (i-C6H5-C=CHBr), 132.6 (o-C6H5-C ), 135.7, 135.8,
136.21, 136.24, 136.3, 136.6 (i-C6H5-C=C=C= and C=CHBr), 147.4,
148.4 ppm (C=C=C=C); MS (Maldi, DCTB): m/z: 532 [M]+.
ꢁ
(C C), 100.4 (CBr2), 128.4, 128.7, 128.7 (o-, m-, p-C6H5), 130.7 (i-C6H5),
137.2 ppm (C=CBr2); MS (DCI/NH3): m/z: 564 [M+NH4]+, 546 [M+H]+
; HRMS (DCI/CH4): m/z calcd for C18H10Br4: 541.7516 [M]+; found:
541.7505.
Synthesis of 24: Dibromo olefin precursor 28 (0.300 g, 0.690 mmol),
freshly activated zinc (0.045 g, 0.690 mmol; washed with diluted HCl)
and distilled DMF (2 mL) were sonicated for 1.5 h. The resulting orange-
brown solution was cooled under stirring at ꢀ788C and freshly purified
CuBr (0.050 g, 0.350 mmol) was added. The temperature was slowly
warmed to RT and stirring was maintained overnight. After treatment
with water, the crude mixture was filtered through Celite and the latter
was washed with diethyl ether. After separation of the two layers, the
aqueous layer was extracted with diethyl ether, and the combined organic
layers were dried over MgSO4 and evaporated to dryness. The residue
was purified by silica gel chromatography (pentane) to give 24 as a
yellow oil in 22% yield. 1H NMR (CDCl3): d=1.087–1.150 ppm (m; Si-
CH-CH3); 19F NMR (CDCl3): d=ꢀ64.13 and ꢀ64.31 ppm (2ꢄs; CF3);
13C{1H} NMR (CDCl3): d=11.13 (s; Si-CH-CH3), 18.44 (s; Si-CH-CH3),
ꢁ
Synthesis of 21: A solution of nBuLi (0.750 mL, 1.83 mmol) was added at
ꢀ1058C to a solution of 14a (0.500 g, 0.92 mmol) in THF (175 mL).
After stirring for 1 h at this temperature, either a suspension of CuCN
(0.082 g, 0.92 mmol) in THF (10 mL) or a solution of CuI·PBu3 (0.361 g,
0.92 mmol) in THF (10 mL), was added at ꢀ908C. The temperature was
allowed to warm slowly to RT and stirring was maintained overnight.
After treatment with saturated aqueous NH4Cl and extraction with dieth-
yl ether, the combined organic layers were washed with brine, dried over
MgSO4, and concentrated under vacuum. After pre-purification by silica
gel chromatography (pentane/chloroform, 98:2 then 9:1), the orange frac-
tion was finally purified by semi-preparative HPLC (petroleum ether/
chloroform, 95:5) to give 21 as an orange oil (ca. 2%). 1H NMR
(CDCl3): d=7.38–7.49 (m, 12H; m- and p-C6H5), 7.60 and 7.62 (2ꢄd, 3J-
ꢁ
98.20 and 98.24 (2ꢄs; C-Si), 101.22 (q, 2J
ACHTNUTRGENNG(U C,F)=49 Hz; C-CF3), 110.03
and 110.50 (2ꢄs; C C-Si), 119.67 and 119.86 (2ꢄq, 1J
(C,F)=275 Hz;
ꢁ
˜
A
E
CF3), 158.54 ppm (s; C=C=C=C); FTIR: n=2946–2894 (Csp3-H), 1614,
1464 (C=C), 1385, 1368, 1278, 1261, 1192 ppm (CF3); UV/Vis: lmax (e)=
373 nm (40250 molLꢀ1 cmꢀ1); MS (DCI/NH3): m/z: 548.1 [M]ꢀ; HRMS
(DCI/CH4): m/z calcd for C28H42F5Si2: 529.2745 [MꢀF]+; found:
529.2755.
4H; o-C6H5-C=); 13C{1H} NMR (CDCl3): d=74.5, 74.6, 80.0, 80.1, 84.1,
ꢁ
ꢁ
ꢁ
84.5 (=C-C C-C ), 86.9, 87.1 (C6H5-C ), 104.5, 104.9 (C=C=C=C),
ꢁ
121.57, 121.60 (i-C6H5-C ), 127.4, 127.5 (o-C6H5-C=), 128.5, 128.6,
128.76, 128.81 (m-C6H5), 129.3, 129.4, 129.6, 129.7 (p-C6H5), 132.6 (o-
ꢁ
C6H5-C ), 135.9, 136.4 (i-C6H5-C=), 148.0, 148.2 ppm (C=C=C=C); UV/
Synthesis of 25: nBuLi (1.97 mL, 4.93 mmol) was added to a solution of
27 (0.750 g, 2.46 mmol) in THF (20 mL) at ꢀ788C. The resulting mixture
was stirred for 20 min at ꢀ788C and for 20 min at RT, then a solution of
26 (0.686 g, 2.46 mmol) in THF (10 mL) was added at ꢀ788C. The mix-
ture was slowly warmed to RT and stirred overnight. After treatment
with saturated aqueous NH4Cl, the aqueous layer was extracted with di-
ethyl ether, and the combined organic layers were washed with brine,
dried over MgSO4, and evaporated to dryness. The residue was purified
by silica gel chromatography (acetone/pentane, 5:95) to give 25 in 71%
yield as a colorless oil, from which the d/l isomer progressively crystal-
lized at RT. 1H NMR (CDCl3): d=1.11 ppm (s, 42H; Si-CH-CH3);
Vis: lmax (e)=499.2 nm (7000 molLꢀ1 cmꢀ1); MS (DCI/NH3): m/z: 451.9
[M]ꢀ.
Characterization of 22: Yield: 5%. 1H NMR (CDCl3): d=7.18 (s, 1H; =
CHBr), 7.37–7.50 (m, 12H; m- and p-C6H5), 7.58–7.63 (m, 2H; o-C6H5-
ꢁ
C ), 7.68–7.71 (m, 2H; o-C6H5-C=CHBr), 7.85–7.89 and 7.97–8.01 ppm
(2ꢄm, 4H; o-C6H5-C ); 13C{1H} NMR (CDCl3): d=74.5, 74.7, 80.0, 84.0,
ꢁ
ꢁ ꢀ ꢁ
ꢀ ꢁ ꢀ
86.5, 86.8 (C C C C), 96.2, 97.2 (=C C C C=), 103.9, 105.6 (C=C=C=
C), 114.7, 114.8 (=CHBr), 121.6 (i-C6H5-C ), 126.35, 126.39 (o- or m-
C6H5-C=CHBr), 127.4, 127.5, 127.6, 127.8 (o-C6H5-C=C=C=), 128.5,
128.6, 128.65, 128.70, 128.73, 128.79, 128.83, 128.9 (o- or m-C6H5-C=
ꢁ
Table 4. X-ray crystallographic data and structural refinement parameters for 2, 6, 7, 18, and 25.
2
6
7
18
25
formula
C38H52Si2
565.00
C22H16
280.35
monoclinic
P21/n
5.7944(3)
7.7835(4)
17.4735(10)
90
94.335(5)
90
785.81(7)
2
C24H22O2
342.42
monoclinic
P21/n
8.0280(4)
12.1192(5)
9.6318(5)
90
92.342(4)
90
936.32(8)
2
C18H12Br2
388.10
orthorhombic
Pbca
15.7508(11)
5.8226(4)
16.2345(12)
90
C28H44F6O2Si2
582.82
tetragonal
P42/n
20.7861(12)
20.7861(12)
15.7939(13)
90
M [gmolꢀ1
]
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
triclinic
¯
P1
9.1361(3)
9.9084(3)
10.2409(4)
93.308(2)
108.6820(10)
98.8660(10)
862.01(5)
1
a [8]
b [8]
g [8]
90
90
90
90
V [ꢃ3]
Z
1calcd
1488.88(18)
4
1.731
5.431
68.40
6823.9(8)
8
1.135
0.1575
56.20
1.088
0.126
70.86
1.185
0.067
52.74
1.215
0.076
64.22
m [mmꢀ1
2qmax [8]
]
crystal size [mm]
T [K]
0.15ꢄ0.20ꢄ0.25
180
0.05ꢄ0.12ꢄ0.20
180
0.12ꢄ0.25ꢄ0.35
180
0.08ꢄ0.25ꢄ0.30
180
0.15ꢄ0.25ꢄ0.25
180
measured refkns
unique reflns
Rint
25264
7112
0.023
5920
1607
0.029
10000
3279
0.027
35046
2305
0.034
67894
8193
0.111
reflns [I>ns(I)]
refinement
5602 (n=3)
F
1156 (n=2)
3110 (n=2)
1326 (n=3)
F
3438 (n=2.2)
F
F2
F2
parameters
181
101
120
91
319
R [I>ns(I)]
0.0367
0.0423
ꢀ0.17/0.44
0.0404
0.1152
ꢀ0.15/0.18
0.0515
0.1383
ꢀ0.19/0.38
0.0242
0.0265
ꢀ0.38/0.55
0.0699
0.0812
ꢀ0.56/0.70
Rw [I>ns(I)]
residual electron density [eꢃꢀ3
]
5098
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Chem. Eur. J. 2011, 17, 5086 – 5100

1,4-Dialkynylbutatrienes
FULL PAPER
19F NMR (CDCl3): d=ꢀ81.46, ꢀ81.48 ppm (2ꢄs; CF3); 13C{1H} NMR
[9] a) R. E. Martin, U. Gubler, C. Boudon, V. Gramlich, C. Bosshard,
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(CDCl3): d=10.9 (s; Si-CH-CH3), 18.3 (s; Si-CH-CH3), 64.3 (q, 2J
ACTHNUTRGNE(NUG C,F)=
ꢁ
ꢁ
ꢁ
37 Hz; C-CF3), 78.8 (s; C-C C-C), 91.2 (s; Si-C C), 97.3 (s; Si-C C),
121.6 ppm (q, 1J
[M+NH4]+.
ACHTUNGTRENNUNG(C,F)=285 Hz; CF3); MS (DCI/NH3): m/z: 600.2
Synthesis of 28: PPh3 (1.743 g, 6.64 mmol) was added to a solution of
CBr4 (1.072 g, 3.23 mmol) in CH2Cl2 (40 mL). After stirring at RT for
15 min, this orange mixture was added to a solution of 26 (0.500 g,
1.80 mmol) in CH2Cl2 (20 mL) at ꢀ788C. The mixture was warmed
slowly to RT and stirred overnight. After evaporation of the solvent
under vacuum, the residue was extracted three times with CH2Cl2/pen-
tane (1:5). The product was finally purified by silica gel chromatography
(pentane) to finally give 28 as a white solid in 82% yield. 1H NMR
(CDCl3): d=1.27 ppm (m, 21H; Si-CH-CH3); 19F NMR (CDCl3): d=
ꢀ60.09 ppm (CF3); 13C{1H} NMR (CDCl3): d=11.0 (s; Si-CH-CH3), 18.5
ꢁ
ꢁ
(s; Si-CH-CH3), 98.9 (s; C-Si), 106.4 (s; C C-Si), 108.5 (s; CBr2), 120.5
(q, 1J(C,F)=276 Hz; CF3), 122.8 ppm (q, 2J
ACHTUNGTRENNUNG ACHTUNGTNER(NUGN C,F)=36 Hz; C=CBr2); MS
(DCI/NH3): m/z: 433.9 [M+H]ꢀ. HRMS (DCI/CH4): m/z calcd for
C14H21SiBr2F2: 412.9771 [MꢀF]+; found: 412.9747.
X-ray crystallographic structure determination of 2, 6, 7, 18, and 25: Data
collection and refinement parameters are given in Table 4. Intensity data
were collected with an Oxford Diffraction Xcalibur or a Bruker Apex2
instrument equipped with an Oxford Cryosystems Cryostream Cooler
Device, using a graphite-monochromated MoKa radiation source (l=
0.71073 ꢃ). Structures were solved by direct methods using SIR92 or
SIR2004, and refined by full-matrix least-squares procedures using the
PC version of CRYSTALS or WINGX software. Scan modes F and W
were used; absorption corrections were obtained in a multiscan mode.
Atomic scattering factors were taken from the International Tables for
X-ray Crystallography. All non-hydrogen atoms were refined anisotropi-
cally. For compound 25, because the crystal was weakly diffracting, the
data were of poor quality and some constraints were applied. Hydrogen
atoms were located in a difference map and repositioned geometrically,
then refined using a riding model. CCDC-793457 (18), 793458 (2), 793459
(25), 793460 (7), and 793461 (6), contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
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from
The
Cambridge
Crystallographic
Data
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www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
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The authors wish to thank the Centre National de la Recherche Scientifi-
que and the Ministꢅre de lꢀEducation Nationale, de la Recherche et de la
Technologie for financial support. Thanks are due to the Paul Sabatier
University and the French Ministery of Research for the Ph.D. fellowship
of L.L.
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[27] The steric hindrance of an organic substituent R can be measured
by a cone angle q(R) the definition of which was validated by an ac-
curate correlation with the experimental Duboisꢀ steric parameters
for saturated groups (E’S(R)=log[k(R)/k(Me)], in which k(R) is the
ꢀ
ꢀ
rate constant of the reaction R CO2H+MeOH!R CO2Me+H2O,
which is assumed to be under the steric control of R according to
the Taft–Ingold hypothesis). Formal application of this definition
gives q(Ph)=52.78 and qACHTNUTRGNE(NUG CF3)=58.48, which are indeed close values
(for comparison, q(Me)=29.28). See: R. Chauvin, H. B. Kagan,
Chirality 1991, 3, 242, and references therein.
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Chem. Eur. J. 2011, 17, 5086 – 5100
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5099

V. Maraval, R. Chauvin et al.
[29] First example: R. Kuhn, H. Krauch, Chem. Ber. 1955, 88, 309; for a
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119, 4415; Angew. Chem. Int. Ed. 2007, 46, 4337.
[31] a) P. A. Morken, N. C. Baenziger, D. J. Burton, P. C. Bachand, C. R.
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[32] Preparation of 24 was also envisioned using nBuLi and a copper salt
(CuI·PBu3) as previously described for the synthesis of 2. In one
case, the butatriene 24 could then be detected by NMR analysis as a
minor product. However, in spite of many attempts, this result could
not be reproduced. In most cases, the dibromo precursor 28 was re-
covered, indicating that the intermediate lithium derivative was not
formed.
Received: September 27, 2010
Revised: January 12, 2011
Published online: March 22, 2011
5100
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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 5086 – 5100